Concussion, or mild traumatic brain injury (MTBI), is the most common and least serious type of traumatic brain injury. A milder type of diffuse axonal injury, concussion involves a transient loss of mental function. It can be caused by acceleration or deceleration forces, or by a direct blow. Concussion is generally not associated with penetrating head trauma.

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The brain floats within the skull surrounded by cerebrospinal fluid (CSF), one of the functions of which is to protect the brain from normal light "trauma", e.g., being jostled in the skull by walking, jumping, etc., as well as mild head impacts. More severe impacts or the forces associated with rapid acceleration/deceleration may not be absorbed by this cushion.

Concussion is considered a type of diffuse brain injury (as opposed to focal brain injury), meaning that the dysfunction occurs over a more widespread area of the brain.

Excitatory neurotransmitters are released as the result of the traumatic injury and cause the brain to enter a state of hypermetabolism which can last for 7 to 10 days.[1] During this time, the brain needs extra nutrients and is especially sensitive to inadequate blood flow.

Damage to cranial nerves and other white matter tracts may be temporary or permanent.[2] Concussion may be a diffuse injury affecting all parts of the brain, caused by physical trauma that alters neuronalmetabolism and excitability through molecular commotion. Having a concussion does not mean that the patient does not have another brain injury as well; in fact, more serious brain trauma is almost always accompanied by concussion.[3]

Symptoms of concussion can include a period of unconsciousness, vomiting, confusion, and visual disturbances. Amnesia, the hallmark sign of concussion, can be retrograde amnesia (loss of memories that were formed before the injury) or anterograde amnesia (loss of memories formed post-injury).[1] In concussion, amnesia is much more likely to be anterograde (also called post traumatic amnesia or PTA). Patients with this type of amnesia are unable to create and retain new memories. Amnesia may not become apparent until a day to a week after the injury. A common example in sports concussion is the quarterback who was able to conduct all the complicated mental tasks of leading a football team after a concussion, but has no recollection the next day of the part of the game that took place after the concussion.

Patients with concussion may act confused, for example repeatedly asking the same questions, or forgetting where they are. Patients may have focal neurological deficits, signs that a specific part of the brain is not working correctly.[4]

Since concussions may not include damage to the brain's structure, the condition of patients with uncomplicated concussions often either improves or stays the same. But brain damage is a process, and not an event, that may set into motion many different pathological processes. The concussions that result in permanent long term deficits often do get worse over the first few days. A deteriorating level of consciousness may mean that the patient has another problem such as a more severe type of head injury. Similarly, persistent vomiting, worsening headache, ringing in the ears (tinnitus), drowsiness, unequal pupil size, and increasing disorientation are all indicative of a rise in intracranial pressure (ICP).[5] More likely in the typical concussion, the process of axonal injury and damage is progressing. Over the first 72 hours, a stretched or damaged axon may be further damaged or killed by ionic fluctuations.[6]

The most critical mistake for those suffering from concussion is not returning for further medical care and evaluation in the time period of 24 to 72 hours after the concussive event, if the symptoms are getting worse. Athletes, especially intercollegiate or professional athletes, would typically be followed closely by team athletic trainers during such period. But those injured in accidents may be sent home with no medical person monitoring them unless the situation gets worse. If the patient does not have a clear recollection of the time period after a concussion that occurred one day ago, then he or she is likely suffering from post-traumatic amnesia, and is more likely to have long term or permanent problems.

Grade III involves the symptoms above, as well as retrograde amnesia and unconsciousness for less than five minutes.[7]

Grade IV involves all of the above symptoms, as well as unconsciousness that lasts between 5 and 10 minutes.[7]

Grade V is the same as grade IV, but with unconsciousness lasting longer than ten minutes.

The American Academy of Neurology (AAN) guidelines make it clear that permanent brain injury can occur with either Grade 2 or Grade 3 concussion. Thus, it is clear that subtle brain injury can have permanent consequences if the acute symptoms of the concussion continue for more than 15 minutes.

Loss of consciousness occurs in less than 10% of concussions; in the rest, the patient experiences only a transient reduction in alertness for a short time.[8]

Healthcare providers make the decision about whether to give the patient a CT scan based on indications that incorporate the Glasgow Coma Scale.[9] Additional considerations include:[9]

children younger than 16 years

intoxicated patients

patients with unreliable followup

patients at risk of bleeding

Each grade of concussion comes with different recommendations for patients who play sports:[10][11]

In grade I, the patient may return to contact sports in 1 week.

A patient with a second time grade I concussion may return to play contact sports 2 weeks after being asymptomatic for a week.

In grade II, the patient may return to contact sports in 1 week of being asymptomatic.[11]

A second time grade II may return to play contact sports 1 month after being asymptomatic for a week.

In grade III, the patient may return to contact sports in 1 month.

For a patient with a second time grade III concussion, the season is over.

However, if the patient has repeated concussions after contact sports, grade I x 3, grade II x 2, and especially grade III x 2, then it should be recommended that the season is over and a thorough medical evaluation should be considered mandatory.

In post-concussion syndrome (PCS), concussion symptoms do not resolve for weeks, months, or even years, and the patient may have headaches, light and sound sensitivity, memory and attention problems, dizziness, difficulty with directed movements, clinical depression, and anxiety. Symptoms usually peak 4 to 6 weeks after the concussion, but may go on longer, some even lasting a year or more.[7] Children commonly experience more severe symptoms of postconcussion syndrome than adults do.[7]Physical therapy plus rest is the best recovery technique, and symptoms usually go away on their own.

Multiple small head injuries that daze the patient can also result in cognitive and physical deficits that occur in what is commonly known as dementia pugilistica, or "punch drunk" syndrome, which is associated with boxers.[12]

The most serious potential complication of concussion is second-impact syndrome (SIS). Patients who receive a second blow days or weeks after a concussion, before symptoms from the first concussion have gone away, are at risk of developing the condition. In this very rare syndrome, the brain swells dangerously after a minor blow. No one is certain of the cause of this often fatal complication, but some think the swelling is due to the brain's arterioles' loss of ability to regulate their diameter, and therefore a loss of control over cerebral blood flow. Intracranial pressure rapidly rises, the brain can herniate, and brainstem failure can occur within five minutes.[12]

The Persian physician Muhammad ibn Zakarīya Rāzi was the first to write about concussion as distinct from other types of head injury in the 10th century AD.[13] In the 13th centrury, the physician Lanfranc of Milan described concussion as brain "commotion," also recognizing a difference between concussion and other types of TBI (though many of his contemporaries did not), and discussing the transience of postconcussion symptoms as a result of temporary loss of function from the injury.[13] In the 14th century, the surgeon Guy de Chauliac pointed out the relatively good prognosis of concussion as compared to more severe types of head trauma such as skull fractures and penetrating head trauma.[13] In the 16th century, the term concussion came into use, and symptoms such as confusion, lethargy, and memory problems were described.[13]

Today, a debate exists about whether concussion involves only transient changes in function (i.e. no actual structural damage) or whether the brain is actually injured the same way it is in other types of TBI (albeit to a lesser extent).[13]